In copending application Ser. No. 149,291, filed May 12, 1980, entitled Repeatered, Multi-Channel Fiber Optic Communication Having Fault Isolation System, by P. Casper et al., and assigned to the assignee of the present application there is described a communication network wherein at a transmitter station high data rate digital data signals are multiplexed with additional signals and the resulting sequence is scrambled and transmitted to a remote receiver station. At the receiver station, the incoming sequence is descrambled and demultiplexed for distribution to various individual circuitry. The communication network described in the above-identified application reflects improvements in the development of sophisticated data transmission systems which are designed to handle a greater density of signalling traffic than previous approaches. The digital signals themselves may represent telephone voice signals, video signals, data, etc. that may be interfaced with the network via telephone lines, microwave terminal interface links, etc.
In this network and other approaches that have been proposed it is convenient and often necessary to organize the data into sequential sets or frames (as is done for video transmission system) and provide frame synchronization signals that permit the receiver station to locate the frames and separate or demultiplex individual sets of signals from a steady stream of data. Where the data stream is scrambled, an additional requirement is placed on the receiver to accurately descramble the incoming signal sequence and then decode or demultiplex the descrambled signals into their original form prior to transmission.
Within the prior art, various systems for accomplishing data scrambling and frame synchronization have been proposed wherein pseudo noise (PN) sequences are used either for scrambling or for providing framing information. The prior art also includes systems that empoly self-synchronized data scrambling and descrambling implementations which involve the use of a maximal length PN sequence that is combined with the data stream to be scrambled via a shift register, modulo-two adder configuration to produce the desired scrambled data sequence. At the receiver, the scrambled data is applied to another shift register, modulo-two adder configuration the selected stage connections of which correspond to those of the scrambler, to thereby remove the scrambling sequence created by the shift register, modulo-two adder feedback arrangement at the transmitter. While this type of arrangement is sometimes useful in that it is self synchronized, requiring no external synchronization, it suffers from error multiplication and less thorough data scrambling than externally synchronized schemes. For an illustration of examples of prior art data scrambling and synchronization arrangements that employ PN sequences, attention may be directed to the U.S. Patents to Tarmo U.S. Pat. No. 3,950,616, Frutiger U.S. Pat. No. 4,979,195, Frey et al. U.S. Pat. No. 3,648,237, Tilk U.S. Pat. No. 3,852,534, Reynolds U.S. Pat. No. 3,808,536, and Betts U.S. Pat. No. 3,947,634.
Unfortunately, these conventional approaches do not provide the degree of signal handling capability and scrambling/descrambling accuracy required for high data rate communication systems. In this regard, as mentioned above, self-synchronizing systems introduce error multiplication and do not have highly thorough scrambling capability.